The continued demand for higher levels of access and increased data rates for a variety of wireless applications, from mobile smart phone devices to back haul point-to-point communications, continues to drive research towards high speed, wideband and low cost radios. Frequency division duplexing (FDD), rather than time division multiple access (TDMA) standards, achieve higher speeds. However, in a FDD system, since the transmitter and the receiver share the same antenna and operate at the same time using a duplexer, a non-negligible transmitter leakage signal may appear at the transmitter front-end through the duplexer. For example, in the WCDMA standard, although the duplexer significantly suppresses transmitter leakage (up to 55 dB), the residual attenuated signal still remains at the receiver input. Thus, the transmitter leakage is often the largest blocker present at the receiver input, making a low-noise and low-power linear receiver design very challenging. This leakage problem is exacerbated by applications such as future cognitive and software-defined radios where the duplex band would ideally be kept to a minimum, to improve spectral efficiency. An off-chip surface acoustic wave (SAW) filter is usually connected between the low noise amplifier and downconverter to further suppress transmitter leakage. However, these filters are band specific, prohibiting highly programmable solutions. Moreover, additional discrete filters are area inefficient, and increase cost/power consumption.
Several recent efforts have attempted to attenuate or reduce the effect of transmitter leakage in the receiver signal path. However, these recent approaches tend to utilize an active cancellation path, which is problematic from a noise and power perspective.